U.S. patent application number 09/750798 was filed with the patent office on 2001-12-06 for method for manufacturing organic electroluminescence display device.
This patent application is currently assigned to NEC CORP. Invention is credited to Tanaka, Taizou.
Application Number | 20010049153 09/750798 |
Document ID | / |
Family ID | 18504653 |
Filed Date | 2001-12-06 |
United States Patent
Application |
20010049153 |
Kind Code |
A1 |
Tanaka, Taizou |
December 6, 2001 |
Method for manufacturing organic electroluminescence display
device
Abstract
In a method for manufacturing an organic EL element, a
low-resistance conductive layer is formed on an electrode of an
organic EL element formed on a substrate in a state in which the
substrate is at a temperature not higher than the freezing point so
that no abnormal current nor short circuit occurs on the organic EL
element when applying a voltage to it.
Inventors: |
Tanaka, Taizou; (Tokyo,
JP) |
Correspondence
Address: |
HELFGOTT & KARAS P.C.
6th Floor
Empire State Building
New York
NY
10118-0110
US
|
Assignee: |
NEC CORP
|
Family ID: |
18504653 |
Appl. No.: |
09/750798 |
Filed: |
December 28, 2000 |
Current U.S.
Class: |
438/29 ;
257/E27.111; 438/22 |
Current CPC
Class: |
H01L 51/5221 20130101;
H01L 27/1214 20130101; H01L 27/12 20130101; H01L 51/56 20130101;
H01L 27/3248 20130101; H01L 51/5231 20130101 |
Class at
Publication: |
438/29 ;
438/22 |
International
Class: |
H01L 021/00; H01L
021/84 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 1999 |
JP |
11-374917/1999 |
Claims
What is claimed is:
1. A method for manufacturing an organic EL element, comprising; a
process of forming a first electrode on a substrate, a process of
forming a light-emission layer on said first electrode, a process
of a second electrode on said light-emission layer, a process of
forming a conductive layer on said second electrode in a state in
which said substrate is cooled to a lower temperature than the
temperature when forming said second electrode.
2. A method for manufacturing an organic EL element according to
claim 1, wherein; said substrate temperature at the time of cooling
said substrate is set in a range of 0 to -60.degree. C.
3. A method for manufacturing an organic EL element according to
claim 1, wherein; said temperature when forming said second
electrode is a room temperature of 15 to 30.degree. C., and during
formation of said conductive layer a material for said conductive
layer is vacuum-evaporated on said second electrode as cooling said
substrate to a temperature not higher than the freezing point by
jetting a coolant gas onto said substrate side.
4. A method for manufacturing an organic EL element according to
claim 1, wherein; a process of forming a hole transport layer
between said first electrode and said light-emission layer is
provided.
5. A method for manufacturing an organic EL element according to
claim 1, wherein; a process of forming a hole injection layer
between said first electrode and said light-emission layer is
provided.
6. A method for manufacturing an organic EL element according to
claim 5, wherein; a process of forming a hole transport layer
between said hole injection layer and said light-emission layer is
provided.
7. A method for manufacturing an organic EL element according to
claim 1, wherein; a process of forming an electron transport layer
between said light-emission layer and said second electrode is
provided.
8. A method for manufacturing an organic EL element according to
claim 7, wherein; a process of forming a hole injection layer
between said light-emission layer and said first electrode is
provided.
9. A method for manufacturing an organic EL element according to
claim 8, wherein; a process of forming a hole transport layer
between said light-emission layer and said hole injection layer is
provided.
10. A method for manufacturing an organic EL element according to
claim 1, wherein; a process of forming a thin film transistor on
said substrate before forming said first electrode and a process of
connecting an electrode of said thin film transistor and said first
electrode to each other are provided.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for manufacturing
an organic electroluminescence (EL) display device having organic
EL elements, and more particularly to a method for forming a
low-resistance conductive layer on one of electrodes of an organic
EL element.
[0003] 2. Description of the Prior Art
[0004] An organic electroluminescence (EL) element utilizes a
phenomenon in which holes injected from an anode and electrons
injected from a cathode are recombined in a light-emission layer
having a luminescence capability and light is emitted when they are
deactivated from an excited state. These studies are improving the
efficiency of light emission by making a light-emission layer be of
a multilayer structure and started from paying attention to a high
luminescence quantum efficiency and a molecular structure capable
of being variously designed of organic compounds, but have only
provided too low brightness and efficiency of light emission to put
their organic EL elements to practical use.
[0005] After that, however, Tang and others found that the
performance was remarkably improved by adopting a layered structure
combined with a material having an excellent capability for
transporting holes (hereinafter referred to as a hole transport
layer) instead of a conventional structure having only a
light-emission layer interposed between electrodes (Applied Physics
Letter, Vol.51, pp.913, 1987).
[0006] Taking this opportunity, the studies have been concentrated
in techniques based on fully separating functions into a layer
having a role of injecting holes (a hole injection layer) and a
layer having a role of transporting electrons (an electron
transport layer) and the like, and the techniques are spurred to be
put to practical use as display devices in conjunction with organic
materials more improved in performance. In recent years, materials
providing emission of the three primary colors, namely, red, green
and blue required for a full color display device in high
brightness and in high efficiency can be obtained and carrier
transport materials suitable for the respective materials are being
selected.
[0007] A method for driving an organic EL element capable of being
used for a unit or pixel of a display device of various kinds of
industrial equipment can follow a liquid crystal driving method.
That is to say, a passive (matrix) driving method depending on the
multiplex performance of elements, an active matrix driving method
to arrange and attach a switching element for each pixel, and the
like can be mentioned.
[0008] However, since an organic EL element has a property which
emits light by passing carriers, namely, flowing current inside it,
its electric characteristics are particularly more important in
comparison with a liquid crystal display device. The reason is
described in the following.
[0009] FIG. 1 shows a process of luminescence of a general organic
EL element. In FIG. 1, an anode 1, a hole injection layer 22, a
hole transport layer 21, a light-emission layer 20 and an electron
transport layer 23 are stacked in order, and holes and electrons
injected by an intense electric field from the anode and cathode
must hop over several organic functional layers having some
potentials before they are recombined.
[0010] It is desirable that each organic layer has a band structure
partitioned by ideal potentials as shown in FIG. 1, but since
actually each organic layer includes a film being liable to
crystallize or put its structure out of order due to change in
temperature, a current leak path is generated and a leak current is
taken out to the outside as an abnormal current in the element.
[0011] And from the same reason, an abnormal current is generated
also when the element is driven by an inverse bias. This means that
so long as an electric field is applied to an element, a leak
current is generated regardless of whether or not the element emits
light, and this becomes a cause to remarkably degrade the quality
of the element as a display device.
[0012] As a method for suppressing such an abnormal current,
Japanese Patent Laid-Open Publication No. Hei 9-102,395 discloses a
method of using aluminum as a material for a cathode. However, this
method has an insufficient effect on suppression of an abnormal
current. And Japanese Patent Laid-Open Publication No. Hei
9-245,965 discloses a technique related to an anode characterized
in that its surface roughness is a maximum of 5 nm or less, but
this technique reveals a problem of a high cost required for
polishing the surface of an anode and a problem of
reproducibility.
SUMMARY OF THE INVENTION
[0013] As a result of diligent studies in consideration of the
above-mentioned problems, an object of the present invention is to
find and provide a method for manufacturing an organic EL display
device to generate neither abnormal current nor short circuit in an
organic EL element of it when a voltage is applied to it.
[0014] In order to attain the above object, the present invention
provides a method for manufacturing an organic EL display device
having an organic EL element having an organic layer which is
composed of at least one layer on a substrate and which is
interposed between a first electrode provided on said substrate
side and a second electrode provided on the other side, said method
being characterized by forming a metal layer or a metal oxide layer
in a state where the substrate is cooled in an area including at
least the second electrode of the organic EL element on the second
electrode.
[0015] And in the above method for manufacturing an organic EL
display device, as the structure of an organic EL element
comprising a first electrode, a second electrode and a
low-resistance conductive layer of a metal layer or a metal oxide
layer, the following structures (1) to (7) obtained by combining
the respective layers can be conceived.
[0016] The above organic EL element is characterized by one of the
following structures:
[0017] (1) first electrode/light-emission layer/second
electrode/low-resistance conductive layer;
[0018] (2) first electrode/hole transport layer/light-emission
layer/second electrode/low-resistance conductive layer;
[0019] (3) first electrode/hole injection layer/light-emission
layer/second electrode/low-resistance conductive layer;
[0020] (4) first electrode/hole injection layer/hole transport
layer/light-emission layer/second electrode/low-resistance
conductive layer;
[0021] (5) first electrode/light-emission layer/electron transport
layer/second electrode/low-resistance conductive layer;
[0022] (6) first electrode/hole injection layer/light-emission
layer/electron transport layer/second electrode/low-resistance
conductive layer;
[0023] (7) first electrode/hole injection layer/hole transport
layer/light-emission layer/electron transport layer/second
electrode/low-resistance conductive layer.
[0024] The first feature of the present invention described above
is in that a process of manufacturing an organic EL display device
includes a process of forming as a film an organic EL element at a
room temperature of 15 to 30.degree. C. or at a temperature of 1 to
14.degree. C. not higher than the room temperature and additionally
a process of forming a low-resistance conductive layer on the
surface including the second electrode of the organic EL element as
keeping the substrate cooled to a low temperature not higher than
the freezing point. It is desirable that the substrate is cooled to
a temperature of 0 to -60.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a cross-sectional view of a schematic structure
for explaining operation of an organic EL element according to a
conventional example.
[0026] FIG. 2A to FIG. 2E are schematic cross-sectional views for
explaining a process of forming an organic EL element according to
the present invention.
[0027] FIG. 3A and 3B are schematic figures for explaining a state
of formation and an effect of action of a low-resistance conductive
layer of the present invention.
[0028] FIG. 4 is a schematic diagram of the structure of an
apparatus for forming a low-resistance conductive layer, said
apparatus being used in the present invention.
[0029] FIG. 5 is a schematic cross-sectional view showing another
layered structure of an organic EL element according to the present
invention.
[0030] FIG. 6A to FIG. 6F are further other layered structures of
organic EL elements according to the present invention.
[0031] FIG. 7 is a characteristic diagram for comparing in leak
current an embodiment of the present invention and a comparative
example with each other.
[0032] FIG. 8 is a schematic cross-sectional view of the structure
of an example in which an organic EL element of the present
invention is formed on a TFT substrate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] A basic manufacturing process according to the present
invention is described with reference to FIG. 2 in the
following.
[0034] As shown in FIG. 2A, a transparent electrode film 10 of ITO
or the like as an anode is formed on a transparent substrate 101 of
a glass plate or the like, and is cleaned by means of an
ultraviolet rays and ozone cleaning process or the like.
[0035] Next, a substrate having a transparent electrode is prepared
by etching the transparent electrode film 10 so as to become a
specified pattern.
[0036] Next, as shown in FIG. 2B, a hole transport layer 21 to be
formed out of .alpha.-NPD
(N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,- 4'-diamine)
and the like is formed on the transparent electrode film 10 by
means of a vacuum evaporation process.
[0037] Following this, as shown in FIG. 2C, a light-emission layer
20 to be formed out of Alq3 (tris (8-quinolinoleate) aluminum
complex) and the like is formed on the hole transport layer 21. It
is desirable that these organic layers 20 and 21 are all formed at
a room temperature of about 30.degree. C.
[0038] Next, as shown in FIG. 2D, a cathode film 30 of an
aluminum-lithium alloy or the like to be the cathode of an organic
EL element is formed on the light-emission layer 20 by means of a
vacuum evaporation process.
[0039] A manufacturing process itself up to here is substantially
the same as a conventional organic EL element manufacturing
process.
[0040] Next, as shown in FIG. 2E, a low-resistance conductive layer
40 of aluminum or the like is formed on the cathode film 30, and
the present invention is characterized in that a process of forming
it is performed while cooling the substrate.
[0041] A method of cooling a substrate on which an organic EL
element is formed is described later with reference to FIG. 4, but
briefly speaking, a low-resistance conductive layer 40 is
evaporated on a cathode layer 30 in a state in which a substrate
101 is cooled by bringing the reverse side of the substrate 101
into contact with a substrate cooling means 300 such as a coolant
reservoir or the like in a vacuum chamber.
[0042] As capabilities required for the low-resistance conductive
layer, there are to further reduce an insufficient resistance of
the second electrode 30 and to prevent moisture in the air from
entering an organic layer. As materials adapted to these
requirements, it is possible to mention aluminum, copper, silver
and the like. In this case the same material as a material for the
second electrode may be used. However, it is desirable that a
substrate is at a room temperature of 15 to 30.degree. C. when a
second electrode 30 is formed, and it is desirable that a substrate
is in a state in which it is cooled to a temperature not higher
than the freezing point when a low-resistance conductive layer 40
is formed. The reason is described in the following.
[0043] The work function of the interface between an organic layer
and a second electrode 30 (the interface between an electron
transport layer 23 and a second electrode layer 30 in this case)
determines the height of an electron injection barrier of an
organic EL element and governs the performance of the element. In
other words, the electron injection barrier results in being liable
to change due to variation of the work function of a thin film of
the second electrode layer 30. It is known that the work function
of a metal material is determined by how the metal is deposited on
a substrate (H. Ishida and others, Phis. Rev. B36, 4510 (1987)),
and a metal film changes its structure depending on the temperature
of its substrate. Therefore, a second electrode 30 of the present
invention can provide a conventional work function by forming the
second electrode layer in a state in which the substrate is kept at
a room temperature.
[0044] On the other hand, since a low-resistance conductive layer
40 is not in contact with an electron transport layer 23 and needs
to reduce its resistance, namely, increase the thickness of the
conductive layer of metal or the like, even if the substrate is
cooled the performance of the organic EL element itself is not
deteriorated.
[0045] Next, the reason why a substrate is cooled when a metal film
is formed as a low-resistance conductive layer 40 in the present
invention is described (see FIG. 3).
[0046] In case that an organic thin film 202 is formed on a
supporting substrate 201 at a room temperature of 30.degree. C.
(FIG. 3A), the organic thin film 202 has some ruggedness on the
surface of it but does not have such a pin hole that it penetrates
the thin film itself formed on it. However, in case that
high-energy particles such as conductive particles 203 and the like
come flying newly onto this thin film to deposit on the organic
thin film 202 (see FIG. 3B), a part of the cohesive energy of metal
(ordinarily, heat energy 204) is given to the organic thin film 202
to be the ground. This energy is changed into crystallization 205
of the organic thin film 202 to put the structure of the thin film
out of order, and may result in generating a through hole in the
organic thin film 202. Hereupon, if the supporting substrate is a
conductive substrate (for example, an ITO substrate or the like), a
short circuit results in occurring between this substrate and the
metal thin film. Such a phenomenon occurs more remarkably with the
increase of the number of metal particles, concretely, the film
formation rate or the film thickness.
[0047] Thereupon, it is effective to suppress crystallization of
the organic thin film 202 and suppress occurrence of pinholes by
cooling the substrate when forming the low-resistance metal layer
40 by means of a technique of the present invention. Further, from
another reason, in case of forming an organic EL element on a TFT
(thin film transistor), if an impurity is mixed into the drain
electrode or the source electrode of the thin film transistor, the
TFT element may be erroneously operated or short-circuited. In case
of forming a metal film, a metal oxide film or the like on a drain
electrode or a source electrode, ions inside the metal or metal
oxide are thermally diffused due to a temperature rise when the
film is formed, and a failure as described above may occur. In
order to suppress this, it is effective to cool the substrate. On
the other hand, there is a method of cooling a substrate when
forming an organic layer (Japanese Patent Laid-Open Publication No.
Hei 4-12,919), but this case is not preferable since it brings a
bad effect that the carrier mobility of the organic layer is
remarkably degraded and the luminescence characteristic of an EL
element is deteriorated.
[0048] Next, a method of cooling a substrate on which an organic EL
element is formed is described with reference to FIG. 4. In a
vacuum chamber 301, the organic EL element side is covered with a
shadow mask 303 arranged close to a substrate 101 having an organic
EL element formed on it, and a low-resistance material is
evaporated from an evaporation source 311 of the low-resistance
material on the bottom. The substrate 101 is cooled from the
reverse side of it by a coolant reservoir 304. In the coolant
reservoir 304, a coolant is circulated through a coolant tube as
shown by the flow 309 of the coolant from a circulator 308 provided
with a temperature controller 307. And as shown by the flow 306 of
a gas from a mass flow controller 305, the substrate 101 is cooled
by flowing a given gas between the substrate 101 and the coolant
reservoir 304.
[0049] This substrate cooling method is performed by installing a
coolant reservoir 304 for storing a coolant in it inside a vacuum
chamber 301 provided with at least one evaporation source 311. The
evaporation source 311 may be of a resistance heating type, and may
be adapted to various film forming types such as sputtering, ion
plating, CVD and the like. This coolant reservoir 304 is connected
to a circulator 308 for circulating a coolant and a temperature
controller 307 for keeping the coolant at a fixed temperature. A
substrate 101 is placed so as to be in contact with this coolant
reservoir 304 and the coolant reservoir 304 is cooled by means of
the temperature controller 307 until the temperature of the coolant
reservoir 304 reaches a specified temperature.
[0050] At this time, in case that a substrate 101 is high in
thermal conductivity like a Si wafer, it is more effective to the
substrate cooling to interpose a sheet being high in thermal
conductivity (for example, an indium sheet, a copper sheet, an
aluminum sheet or the like) in order to improve the contact between
the substrate 101 and the coolant reservoir 304. And in case that a
substrate 101 is low in thermal conductivity like a glass
substrate, the cooling effect can be improved by making a gap of
several microns to several hundred microns between the substrate
101 and the coolant reservoir 304, and flowing a gas (inert gas,
nitrogen or the like) between the substrate 101 and the coolant
reservoir 304. It is desirable that such a gas is introduced into
the vacuum chamber 301 through such a device capable of controlling
the flow rate of a gas as a mass flow controller or the like
provided outside the chamber 301. It is preferable that for the
flow rate of a gas, the pressure inside the vacuum chamber 301 is
set at a pressure of 2.times.10-2 Pa or less in consideration of
the average free path of evaporated particles at the time of
forming a film.
[0051] According to such a technique, it is enough to heat the
evaporation source 311 at a stage in which the substrate 101 has
been cooled and evaporate a low-resistance material on the surface
electrode of an organic EL element formed on the substrate 101, but
when the evaporation source 311 and the substrate 101 are too close
to each other, the substrate 101 is liable to rise in temperature
due to heat radiation from the evaporation source 311 and therefore
it is desirable that they have an interval of at least 100 mm or
more between them. And a conveyance mechanism (not illustrated)
capable of moving a substrate 101 to another vacuum chamber or the
like may be provided.
[0052] A process of cooling a substrate 101 can be realized by the
method described above, and another cooling method utilizing
Peltier effect or the like is also effective.
[0053] According to such a manufacturing method, an organic EL
element having a layered structure of the first electrode 10
(anode)/hole transport layer 21/light-emission layer 20/second
electrode 20 (cathode)/low-resistance conductive layer 40 can be
obtained.
[0054] As a method of cooling a substrate, a cooling method
utilizing Peltier effect or the like is also effective.
[0055] Although the thickness of a low-resistance conductive layer
40 is optional, in case that the temperature of a substrate rises
by 20.degree. C. or more due to heat radiation from the evaporation
source 311, since the surface itself of the substrate rises in
temperature higher than that temperature, it is more preferable to
once close a shutter for forming a film or turn power off and wait
until the substrate temperature comes again to fall in a specified
temperature range. The cooling temperature is preferably not higher
than the freezing temperature and particularly preferably in a
range of 0 to -150.degree. C. When it is not higher than
-70.degree. C., migration at the time of forming a low-resistance
metal layer 407 is suppressed and its film structure is changed and
its metallic gloss may be lost, and therefore the cooling
temperature is more preferably in a range of 0 to -60.degree.
C.
[0056] Although in an organic EL element described above a cathode
30 has been formed directly on a light-emission layer 20, as shown
in FIG. 5 an electron transport layer 23 of tris (8-quinolinoleate)
aluminum complex or the like is formed on the light-emission layer
20, and a cathode layer 30 of an aluminum-lithium alloy or the like
and a low-resistance conductive layer 40 of aluminum or the like
are formed thereon, and thereby an organic EL element having a
layered structure of the first electrode 10 (anode)/hole transport
layer 21/light-emission layer 20/electron transport layer 23/second
electrode (cathode) 30/low-resistance conductive layer 40 can be
obtained.
[0057] Although the thickness of a low-resistance conductive layer
40 is optional, in case that the temperature of a substrate rises
by 20.degree. C. or more due to heat radiation from the evaporation
source 311, since the surface itself of the substrate rises in
temperature higher than that temperature, it is more preferable to
once close a shutter for forming a film or turn power off and wait
until the substrate temperature comes again to fall in a specified
temperature range.
[0058] The cooling temperature is preferably not higher than the
freezing temperature and particularly preferably in a range of 0 to
-150.degree. C. When it is not higher than -70.degree. C.,
migration at the time of forming a low-resistance metal layer 407
is suppressed and its film structure is changed and its metallic
gloss may be lost, and therefore the cooling temperature is more
preferably in a range of 0 to -60.degree. C.
[0059] And in the present invention, in case of forming the layered
structure of an organic EL element by stacking layers in the
inverse order, it is necessary to deposit a metal oxide such as
indium-tin oxide (ITO), indium-zinc oxide (IXO) or the like as a
transparent conductive layer on an organic layer. In this case, a
measure of a mesh electrode or the like is preferably taken for ITO
having a great radiation heat, but it is more preferable to use a
technique or material for forming a film of a specified resistance
value and transmissivity at a room temperature.
[0060] As the structure of an organic EL element according to the
present invention, layered structures as shown in FIG. 6A to 6F can
be conceived in addition to the above-mentioned layered
structures.
[0061] (1) First electrode 10/light-emission layer 20/second
electrode 30 (cathode)/low-resistance conductive layer 40 (FIG.
6A);
[0062] (2) First electrode 10 (anode)/hole injection layer
22/light-emission layer 20/second electrode 30/low-resistance
conductive layer 40;
[0063] (3) First electrode 10 (anode)/hole injection layer 22/hole
transport layer 21/light-emission layer 20/second electrode 30
(cathode)/low-resistance conductive layer 40;
[0064] (4) First electrode 10 (anode)/light-emission layer
20/electron transport layer 23/second electrode 30
(cathode)/low-resistance conductive layer 40;
[0065] (5) First electrode 10 (anode)/hole injection layer
22/light-emission layer 20/electron transport layer 23/second
electrode 30 (cathode)/low-resistance conductive layer 40;
[0066] (6) First electrode 10 (anode)/hole injection layer 22/hole
transport layer 21/light-emission layer 20/electron transport layer
23/second electrode 30 (cathode)/low-resistance conductive layer
40.
[0067] And in these various layered structures of organic EL
elements, each layer may be a mixed layer composed of two or more
kinds of materials and another functional layer may be interposed
between the respective organic layers. Further, the order of
stacked layers may be inverted, and an organic EL element may be
composed so as to be controlled by a switching element such as a
TFT (thin film transistor) or the like.
[0068] Light emission can be obtained from a light-emission layer
by applying a voltage to an organic EL element described above. And
a full-color organic EL display device can be formed by forming a
character and an image through forming pixels and switching
elements in the shape of a matrix, and separating organic EL
elements into each pixel of, for example, red (R), green (G) and
blue (B) and applying an image signal to the respective pixels.
[0069] A material for a hole transport layer 21 used in an organic
EL element according to the present invention is not limited in
particular, and an ordinary compound or new compound to be used as
a conventional hole transport material can be also used.
[0070] As such a hole transport material, there can be mentioned,
for example, an aromatic diamine compound in which 3-class aromatic
amine units such as 1,1-bis(4-ditolyl aminophenyl) cyclohexane and
the like are linked (Japanese Patent Laid-Open Publication No. Sho
59-194,393); aromatic amine which contains two or more 3-class
amines, has nitrogen atoms substituted for two or more fused
aromatic rings and is represented by
4,4'-bis(N-(1-naphthyl)-N-phenylamino) biphenyl (Japanese Patent
Laid-Open Publication No. Hei 5-234,681); aromatic triamine being a
derivative of triphenyl benzene and having a star-burst structure
(U.S. Pat. No. 4,923,774); aromatic diamine such as N,N'-diphenyl-N
or N'-bis(3-methylphenyl) biphenyl-4 or 4'-diamine or the like
(U.S. Pat. No. 4,764,625); .alpha., .alpha., .alpha.',
.alpha.'-tetramethyl-.alpha.,
.alpha.'-bis(4-di-p-tolylaminophenyl)-p-xylene (Japanese Patent
Laid-Open Publication No. Hei 3-269,084); triphenylamine derivative
being three-dimensionally asymmetric on the whole molecule
(Japanese Patent Laid-Open Publication No. Hei 4-129,271); a
compound having pyrenyl groups substituted for plural aromatic
diamine groups (Japanese Patent Laid-Open Publication No. Hei
4-175,395); aromatic diamine having 3-class aromatic amine units
linked by ethylene groups (Japanese Patent Laid-Open Publication
No. Hei 4-264,189); a compound having 3-class aromatic amine units
linked by thiophene groups (Japanese Patent Laid-Open Publication
No. Hei 4-308,688); a benzylphenyl compound (Japanese Patent
Laid-Open Publication No. Hei 4-364,153); a compound having 3-class
amine units linked by fluorene groups (Japanese Patent Laid-Open
Publication No. Hei 5-25,473); a triamine compound (Japanese Patent
Laid-Open Publication No. Hei 5-239,455); bisdipyridyl
aminobiphenyl (Japanese Patent Laid-Open Publication No. Hei
5-320,634); N,N,N-triphenylamine derivative (Japanese Patent
Laid-Open Publication No. Hei 6-1,972); aromatic diamine having a
phenoxadine structure (Japanese Patent Laid-Open Publication No.
Hei 7-138,562); diaminophenyl phenantolidine derivative (Japanese
Patent Laid-Open Publication No. Hei 7-252,474); a hydrazone
compound (Japanese Patent Laid-Open Publication No. Hei 2-311,591);
a silazane compound (U.S. Pat. No. 4,950,950); silanamine
derivative (Japanese Patent Laid-Open Publication No. Hei
6-49,079); phosphamine derivative (Japanese Patent Laid-Open
Publication No. Hei 6-25,659); a quinacridon compound; and the
like.
[0071] The patent publications and the like contained in
parentheses ( ) following the names of materials for a hole
transport layer are the patent publication numbers introducing
these material names. This is also the same as the following.
[0072] And these compounds may be used independently and may be
used being mixed with one another according to need.
[0073] Further, additionally to the above-mentioned materials, as
materials for a hole transport layer, there can be mentioned
polyvinyl carbazole or polysilane (Appl. Phys. Lett., Vol. 59, pp.
2760, 1991); polyphosphozene (Japanese Patent Laid-open Publication
No. Hei 5-310,949); polyamide (Japanese Patent Laid-Open
Publication No. Hei 5-310,949); polyvinyl triphenyl amine (Japanese
Patent Laid-Open Publication No. Hei 7-53,953); polymer having a
triphenyl amine skeleton (Japanese Patent Laid-Open Publication No.
Hei 4-133,065); polymer having triphenyl amine units linked by
methylene groups and the like; and polymer such as polymethacrylate
and the like containing aromatic amine.
[0074] As materials for a hole injection layer, copper
phthalocyanine and the like can be mentioned.
[0075] And as materials for a light-emission layer 20, there can be
mentioned anthracene, pyrene, Tris(8-quinolinoleate) aluminum
complex and its derivative known as luminescent materials from the
past, and in addition to them, bis-stylylanthracene derivative,
tetraphenyl butadiene derivative, coumarin derivative, oxadiazole
derivative, distylyl benzene derivative, pyrrolopyridine
derivative, perinone derivative, cyclopentadiene derivative,
oxazole derivative, thiadiazolopyridine derivative, perinon
derivative, and polymer such as polyphenylene vinylene derivative,
polyparaphenylene derivative, polythiophene derivative and the
like.
[0076] And for the purpose of improving the efficiency of light
emission or making longer the life of light emission, a
light-emission layer may have a very small amount of impurity
intentionally mixed in it, and concretely the impurity is selected
from rublene, quinacridon derivative, phenoxazone 660,
dicyanomethylene stylyl pyrane, perinon, perirene, coumarin
derivative, dimethyl aminopyradine carbonitrile, pyradine
dicarbonitrile derivative, Nile Red, rhodamine derivative and the
like.
[0077] The performance required for material used for an electron
transport layer 23 is to efficiently transport electrons injected
from a cathode. For this purpose, the material should be high in
electron mobility and electron affinity, and excellent in film
forming ability.
[0078] As materials to meet such a condition, there can be
mentioned oxine complex such as tris(8-quinolinoleate) aluminum
complex and the like, periren derivative, perinon derivative,
naphthalene derivative, coumarin derivative, oxadiazole derivative,
phenanthroline derivative and the like.
[0079] Formation of a thin film of these organic materials
described above is performed mainly by a vacuum evaporation method,
but can be also performed by an electron beam evaporation method, a
sputtering method, a molecule stacking method and a coating method
using a solvent. In case of such a coating method, an organic
material can be coated by being dissolved or dispersed in a solvent
together with polyvinyl chloride, polycarbonate, polystyrene,
poly(N-vinyl carbazole), polymethyl methacrylate, polybutyl
methacrylate, open-ring polymer of norbornene derivative,
polyester, polysulfone, polyphenylene oxide, polybutadiene,
hydrocarbon resin, ketone resin, phenoxy resin, polyamide, ethyl
cellulose, ABS resin, phenol resin, silicone resin, epoxy resin and
the like.
[0080] And in the present invention, as shown in FIG. 5, a first
electrode 10 needs to be transparent in case of taking out light
from the supporting substrate 101 side. It is formed out of a
conductive metal oxide such as tin oxide, indium oxide, indium-tin
oxide (ITO), indium-zinc oxide (IXO) or the like. Or such metal as
gold, silver, chromium, aluminum or the like may be stacked and
formed as a bus electrode on a part of ITO into a layered
structure; or a conductive inorganic material such as copper
iodide, copper sulfide or the like, or polythiophene, polypyrrole,
polyaniline, polyphenylene vinylene, polyphenylene or polyethylene,
or these conductive polymer processed by doping may be used as a
bus electrode and formed with ITO into a layered structure.
Preferably, ITO glass, Nesa glass or IXO glass is used.
[0081] Since the resistance of an anode is enough if it can supply
a sufficient current for light emission of an element, it is not
limited in particular, but from the viewpoint of power consumption
of the element it is desirable that the anode has a low
resistance.
[0082] Next, since a second electrode 30 must make an electron
transport layer efficiently transfer electrons to a light-emission
layer, the adhesion of the electrode to a material adjacent to it,
the adjustment of ionizing potential and the like are needed. And
in order to keep a stable performance for a long-term use, it is
desirable to use a comparatively stable material in the air, but
since a protective film or a proper sealing means can be used, it
is not limited to these. Concretely, it is possible to use metal
such as indium, gold, silver, aluminum, lead, magnesium or the
like, or a simple substance of rare-earth element, alkali metal, or
an alloy of these, and the like. And even metal compounds of
fluoride or oxide of alkali metal can be used in case that their
work function is suitable for injection.
Embodiments
[0083] First Embodiment
[0084] An embodiment according to the present invention is
described with reference to FIG. 5 in the following. In this
embodiment, an ITO film as a transparent electrode 10 was formed so
as to be 100 nm in thickness on a transparent glass substrate 101
by a sputtering method. Its sheet resistance at this time was 10
.omega./.quadrature.. Next, the ITO film was etched so as to form a
specified pattern and thereby a glass substrate with an ITO pattern
was prepared. This substrate was cleaned with pure water and IPA,
and then was cleaned by means of an ultraviolet (UV) and ozone
cleaning process to sufficiently clean the surface of it.
[0085] Next, .alpha.-NPD
(N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-bipheny- l)-4,4'-diamine)
of 100 mg in a boat of molybdenum as a material for a hole
transport layer, and Alq3 (tris(8-quinolinoleate) aluminum complex)
of 100 mg in a boat of molybdenum as a luminescent material were
separately prepared and were set inside a vacuum evaporation
apparatus so that they are different evaporation sources. The
substrate 101 which was previously prepared and had a transparent
electrode formed on it was set inside the same vacuum evaporation
apparatus, and then the inside of the apparatus was exhausted to a
vacuum degree of 2.times.10.sup.-4 Pa, and at the point of time
when the vacuum degree was reached the boat containing .alpha.-NPD
started to be heated.
[0086] The temperature was controlled until the evaporation rate of
the .alpha.-NPD reached a constant rate of 0.3 nm/sec, and then a
shutter provided above the boat was opened to start forming a film,
and at the point of time when a film of 50 nm in thickness was
formed, the shutter was closed and the evaporation was ended.
[0087] In the same manner, a film of Alq3 was formed at a film
formation rate of 0.3 nm/sec to be 55 nm in thickness, and
formation of the organic layer was ended. The organic layer
formation at this time was always performed at a room temperature
of about 30.degree. C. Next, the substrate having this organic
layer formed on the surface of it was set again so that the reverse
surface of it came into contact with a coolant reservoir in the
vacuum chamber, the boats just now used for forming the organic
film were removed, instead of them an aluminum-lithium alloy of 1 g
was put in a tungsten boat and aluminum of 2 g was put in another
tungsten boat, and the inside of the vacuum chamber was exhausted
again. At the point of time when the chamber was exhausted to
4.times.10.sup.-4 Pa, the boat containing the aluminum-lithium
alloy was heated under a condition in which the substrate
temperature is a room temperature, and the boat temperature was
controlled so that an evaporation rate of 0.4 nm/sec could be
obtained. When a stable state was obtained, a shutter provided
above was opened and when an aluminum-lithium alloy film was formed
to be 60 nm in thickness, the film formation was ended, and thereby
an organic EL element having a structure of
ITO/.alpha.-NPD/Alq3/AlLi was obtained.
[0088] Next, in order to form a low-resistance metal film, inside
the vacuum chamber 301, the temperature of the coolant reservoir
was set to the conditions shown in Table 1 (corresponding to No. 3
to 10), and when a specified temperature was obtained, a nitrogen
gas was flowed from between the substrate and the coolant
reservoir. The flow rate of the nitrogen gas at this time was 0.5
SCCM. When the temperature of the substrate came into a
sufficiently stable state, the boat containing aluminum was heated
and its temperature was controlled so that an evaporation rate of
0.7 nm/sec was obtained. When the evaporation rate became stable, a
shutter provided above was opened and when an aluminum film was
formed to be 500 nm in thickness, the film formation was
finished.
1TABLE 1 Occurrence Recti- Substrate of short fication Specific No
temperature circuit ratio resistance 1 Comparative 60 Yes 1.1E3 2.9
example 1 2 Comparative 30 Yes 2.5E9 3.2 example 3 3 Embodiment 0
No 1.1E7 2.0 4 1 -10 No 2.3E7 1.9 5 -20 No 3.6E7 2.2 6 -30 No 6.1E7
3.0 7 -40 No 3.4E7 2.2 8 -50 No 2.3E7 1.9 9 -70 No 1.6E7 0.9 10
-150 No 1.0E7 0.7
[0089] This element was taken out from the vacuum chamber 301 and
the transparent electrode 10 of ITO was connected to the positive
pole of a power source and the low-resistance conductive layer 40
of aluminum was connected to the negative pole of the power source.
An applied voltage was made to vary from 0 V to 20 V to check
existence of a short circuit, but no short circuit was found and
emission of green light was obtained. Next, taking the ITO as an
anode and the aluminum as a cathode, a leak current of the organic
EL element was measured by means of a semiconductor parameter
analyzer. As a result, the rectification ratio when applying a
voltage of +15 V was 1.times.10.sup.7 or more and no leak current
occurred.
[0090] Further, a result of measuring the specific resistance of
the aluminum being the low-resistance electrode showed a
low-resistance value. The result is shown in Table 1 (corresponding
to No. 3 to 10), the result of measurement of an element whose film
was formed at a temperature of -40.degree. C. is shown in FIG.
7.
[0091] From FIG. 7, it is seen that the characteristic of currents
to applied voltages is shown for an embodiment and a comparative
example, even when an inverse bias voltage is applied the current
value is stable and no leak current occurs. And as a result of
observing the light-emission part of each of these elements,
conspicuous dark spots were not found at all.
[0092] Description of a Manufacturing Process
[0093] A manufacturing process according to an embodiment of the
present invention is described referring again to FIG. 2. FIG. 2A
shows a process of forming a thin ITO film to 100 nm in thickness
on a glass substrate 101 and cleaning it.
[0094] As performances required for a transparent electrode ITO 10
to be used in an organic EL element, there are a transmissivity of
80% or more and a sheet resistance of 50 .omega./.quadrature. or
less. Such requirements are adopted for the purpose of making the
efficiency of taking out light as high as possible and reducing the
resistance of wiring. In this case, an ITO film of 100 nm in
thickness meets these requirements. And a UV-ozone cleaning process
is performed for the purpose of removing organic foreign matters
remaining on an ITO substrate.
[0095] Next, FIG. 2B and FIG. 2C respectively show processes of
forming a hole transport layer 21 and a light-emission layer 20 on
the substrate prepared in the process of FIG. 2A. The evaporation
rate is the thickness of a film deposited on a substrate per unit
time, and an organic matter (a hole transport layer 21 or a
light-emission layer 20 in this case) is ordinarily formed into a
film at an evaporation rate of 0.3 to 5 nm/sec. The reason is that
a too fast evaporation rate raises the temperature of an
evaporation source and leads to decomposition of a material. And as
the thickness of an organic film a thickness of several nm to
several hundred nm has been used from the past. The reason is that
a too thin organic film thickness is liable to generate pinholes,
while a too thick film thickness causes movement of carriers
injected into the material to be delayed by impurities or raises
the driving voltage.
[0096] Next, a process shown in FIG. 2D shows a second electrode
forming process for forming the cathode 30 of an organic EL
element. The evaporation rate of a cathode 30 can be selected from
a range of about 0.3 to 5 nm/sec from the same reason as when
forming an organic film. And its film thickness needs to be thick
enough to sufficiently meet the flowing current, but if it is too
thick, a short circuit as described above may occur and therefore a
thickness of about 40 to 100 nm is preferable.
[0097] A process up to here is the same as a conventional organic
EL element manufacturing process.
[0098] Next, a process shown in FIG. 2E is a process having a
feature of the present invention for forming a low-resistance metal
layer as cooling a substrate. A method of cooling the substrate is
as shown in FIG. 4. Although the thickness of a low-resistance
metal layer is optional, in case that the temperature of a
substrate rises by 20.degree. C. or more due to heat radiation from
the evaporation source 311, since the surface itself of the
substrate rises in temperature higher than that temperature, it is
more preferable to once close a shutter for forming a film or turn
power off and wait until the substrate temperature comes again to
fall in a specified temperature range. The cooling temperature is
preferably not higher than the freezing temperature and
particularly preferably in a range of 0 to -150.degree. C. When it
is not higher than -70.degree. C., migration at the time of forming
a low-resistance metal layer 407 is suppressed and its film
structure is changed and its metallic gloss may be lost, and
therefore the cooling temperature is more preferably in a range of
0 to -60.degree. C.
[0099] Second Embodiment
[0100] Organic EL Element of an Inverse Layered Type
[0101] A second embodiment of the present invention is described in
detail with reference to FIG. 5. An aluminum-lithium alloy was
formed on a transparent glass substrate 101 into a film of 100 nm
as a first electrode 10 by means of a sputtering process. Next, the
aluminum-lithium alloy was etched so as to form a specified pattern
to prepare a glass substrate with an aluminum-lithium thin film.
This substrate was cleaned by a UV-ozone cleaning process so that
the surface of it was cleaned sufficiently.
[0102] Next, as a material for a hole transport layer, Alq3
(tris(8-quinolinoleate) aluminum complex) of 100 mg in a boat of
molybdenum as a luminescent material, and .alpha.-NPD
(N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine)
of 100 mg in a boat of molybdenum and additionally copper
phthalocyanine (CuPc) of 100 mg in a boat of molybdenum were set
inside a vacuum evaporation apparatus so that they are,
respectively, different evaporation sources. The substrate 101
which was previously prepared was set inside the same vacuum
evaporation apparatus, and then the inside of the apparatus was
exhausted to a vacuum degree of 2.times.10.sup.-4 Pa, and at the
point of time when the vacuum degree was reached the boat
containing Alq3 started to be heated.
[0103] The temperature was controlled until the evaporation rate of
Alq3 reached a constant rate of 0.3 nm/sec, and then a shutter
provided above was opened to start forming a film, and at the point
of time when a film of 50 nm in thickness was formed, the shutter
was closed and the evaporation was ended. In the same manner, a
film of .alpha.-NPD was formed at an evaporation rate of 0.3 nm/sec
to a thickness of 50 nm, and further in the same manner, a film of
CuPc was formed at an evaporation rate of 0.3 nm/sec to a thickness
of 30 nm, and the organic layer forming process was ended. The
organic layer formation at this time was always performed at a room
temperature. Next, the substrate having this organic layer formed
on it was set again so as to come into contact with a coolant
reservoir of a magnetron sputtering apparatus, and a backing plate
having the IXO set on it was set at a specified position and the
apparatus was exhausted. At the point of time when the chamber was
exhausted to 4.times.10.sup.-4 Pa, an oxygen gas was introduced
into the sputtering apparatus so that the partial pressure of
oxygen becomes 0.02 Pa, and a sputtering process was started by
turning on a power of 0.90 W/cm.sup.2.
[0104] The temperature of the substrate at this time was a room
temperature of about 30.degree. C. When the film thickness became
40 nm, the film formation was ended, and thereby an organic EL
element having an inverse layered structure of
AlLi/Alq3/.alpha.-NPD/CuPc/IXO was obtained.
[0105] Next, the above-mentioned organic EL element was loaded
inside the vacuum chamber 301 shown in FIG. 4, and in order to form
a low-resistance conductive film on the IXO film, the temperature
of the coolant reservoir was set to -40.degree. C., and when its
temperature became a specified temperature, a nitrogen gas was
flowed from between the substrate and the coolant reservoir. The
flow rate of nitrogen at this time was 0.5 SCCM. When the
temperature of the substrate came into a sufficiently stable state,
an oxygen gas was introduced so that the partial pressure of oxygen
became 0.01 Pa, sputtering IXO was started according to the same
technique as described above. When the evaporation rate became
stable, a shutter provided above was opened and when an IXO film
was formed to be 500 nm in thickness, the film formation was
finished.
[0106] This element was taken out from the vacuum chamber 301 and
the IXO film being a low-resistance conductive layer 40 was
connected to the positive pole of a power source and the
aluminum-lithium alloy layer 10 was connected to the negative pole
of the power source. An applied voltage was made to vary from 0 V
to 20 V to check existence of a short circuit, but no short circuit
was found and emission of green light was obtained. Next, taking
the IXO as an anode and the aluminum-lithium alloy layer as a
cathode, a leak current of the organic EL element was measured by
means of a semiconductor parameter analyzer. As a result, the
rectification ratio when applying a voltage of +15 V was
4.times.10.sup.6 or more and no leak current occurred. And as a
result of observing occurrence of dark spots by means of an optical
microscope, conspicuous dark spots were not found at all.
Comparative Example 1
Forming a Low-resistance Metal Layer at a Room Temperature
[0107] Next, a comparative example to said embodiment 1 is
described. A low-resistance metal layer was formed in the vacuum
chamber 301 described in FIG. 4, and an organic EL element was
formed in the same manner as embodiment 1 except that the
low-resistance metal layer 40 was formed at a room temperature of
30.degree. C. without using a coolant at that time.
[0108] The ITO of this element was connected to the positive pole
of a power source and the aluminum was connected to the negative
pole of the power source. An applied voltage was made to vary from
0 V to 20 V to check existence of a short circuit, and an element
having a portion short-circuited and being unable to emit light was
found. Next, taking the ITO of an element being not short-circuited
as an anode and the aluminum as a cathode, a leak current of the
organic EL element was measured by means of a semiconductor
parameter analyzer. As a result, the rectification ratio when
applying a voltage of +15 V was 2.5.times.10.sup.3 and leak current
occurred (FIG. 7). The result was shown in Table 1 (corresponding
to No. 2 comparative example 3 in Table 1). And as a result of
observing the light-emission part of each of these elements by
means of an optical microscope, conspicuous dark spots were not
found at all.
Comparative Example 2
With No Low-resistance Metal Layer
[0109] Next, a comparative example 2 is described. An organic EL
element was formed in the same manner as embodiment 1 except that
no low-resistance metal layer was formed.
[0110] Taking the ITO of this element as an anode and the aluminum
as a cathode, a leak current of the organic EL element was measured
by means of a semiconductor parameter analyzer. As a result, the
rectification ratio when applying a voltage of +415 V was
1.times.10.sup.7 or more and no leak current occurred.
[0111] However, as a result of observing the light-emission part of
each of these elements by means of an optical microscope, the
density of conspicuous dark spots was 17 spots/mm.sup.2, and their
light-emission surfaces were found to be uneven.
[0112] And the specific resistance of an aluminum film measured was
found to be 5.5.times.10.sup.-9 m, and it was higher by about 1
figure than embodiment 1.
Comparative Example 3
Forming a Low-resistance Metal Layer at 60.degree. C.
[0113] Next, comparative example 3 is described. An organic EL
element was formed in the same manner as embodiment 1 except that a
low-resistance metal layer was formed at a substrate temperature of
60.degree. C.
[0114] The ITO of this element was connected to the positive pole
of a power source and the aluminum was connected to the negative
pole of the power source. An applied voltage was made to vary from
0 V to 20 V to check existence of a short circuit, but an element
having a portion short-circuited and being unable to emit light was
found. Next, taking the ITO of an element being not short-circuited
as an anode and the aluminum as a cathode, a leak current of the
organic EL element was measured by means of a semiconductor
parameter analyzer. As a result, the rectification ratio when
applying a voltage of +15 V was 1.1.times.10.sup.3 and a leak
current occurred. The result was shown in Table 1 (corresponding to
No. 1 comparative example 1 in Table 1). And as a result of
observing the light-emission part of each of these elements by
means of an optical microscope, conspicuous dark spots were not
found at all.
[0115] Third Embodiment
[0116] Making an Organic EL Element on a TFT Substrate
[0117] Next, a third embodiment of the present invention is
described with reference to FIG. 8.
[0118] An aluminum (Al) film of 150 nm in thickness was formed by a
resistance heating evaporation process on a cleaned non-alkali
glass (No. 1737 manufactured by Corning, Inc.) substrate 801, and
then a gold (Au) film of 10 nm in thickness was deposited on the
resulting substrate by means of the same technique.
[0119] This was patterned by photolithography and wet-etching for a
gate electrode. On the gate electrode 81, a gate insulating film 82
of silicon nitride was formed to become 200 nm in thickness by
means of a plasma CVD method. Further, a p-Si film 83 was formed on
it to be 60 nm in thickness, and then a SiO, film was patterned
into a specified shape. Using this SiO, film as a mask, phosphorus
was ion-implanted to form a source domain 831 and a drain domain
832.
[0120] And a SiO.sub.2 film was deposited again, and then SiN was
evaporated to form an interlayer insulating film 84.
[0121] Next, the interlayer insulating film was etched so that the
upper portions of the source and drain domains were opened, and
aluminum was evaporated on the whole surface including the opening,
and then the aluminum except the aluminum part being on the opening
and near the wiring part was etched away and the source electrode
and the drain electrode were pattern-formed. Next, the whole
surface was coated with a polyimide film by a spin coating method,
and thereafter the upper parts of the source electrode and the
drain electrode were opened.
[0122] Following this, after aluminum was evaporated all over the
surface, a source electrode wiring portion 851 and a drain
electrode wiring portion 852 were patterned and were coated with a
polyimide film 86, and an opening to expose the source electrode
wiring portion was provided. Further, openings were provided in the
areas where no TFT element was deposited, and an ITO film 10 was
formed to be 150 nm in thickness on these areas by means of a
sputtering method.
[0123] Furthermore, the source electrode of the TFT element and
this ITO film were connected by a metal film of as aluminum or the
like by means of an evaporation method and an etching method. An
organic EL element was deposited on the TFT substrate made in such
a way at a substrate temperature being a room temperature in the
same manner as the embodiment of a layered structure shown in FIG.
5, and an aluminum film of 500 nm in thickness was formed thereon
at a substrate temperature of -40.degree. C. as a low-resistance
metal layer 40. When driving this organic EL element with TFT, it
was found that no short circuit occurred and no TFT erroneously
operated.
Comparative Example 4
In Case of Forming a Low-resistance Metal Layer at a Room
Temperature When Making an Organic EL Element on a TFT
Substrate
[0124] Next, comparative example 4 is described. An organic EL
element with TFT was formed in the same manner as the third
embodiment except that a low-resistance metal layer was formed at a
room temperature when forming the low-resistance metal layer after
forming an organic EL element. When driving this organic EL element
with TFT, it was found that a short circuit occurred and a TFT
erroneously operated.
[0125] As described above, the first advantage of the present
invention is in that it is possible to suppress a short circuit and
a leak current in an organic EL element and reduce the resistance
of an area including the wiring portion. As described above, FIG. 7
is a graph showing the first advantage of the present
invention.
[0126] It is understood that like the embodiment of FIG. 7 a leak
current is suppressed even in case of applying an inverse bias
voltage, by using a manufacturing method of the present
invention.
[0127] And as the second advantage of the present invention, it is
possible to suppress a leak current and simultaneously suppress
occurrence of dark spots. The reason is that a low-resistance
conductive layer has an advantage of preventing moisture from
entering an organic EL element (sealing effect), and that there is
not heating of an organic EL element to be a cause of occurrence of
dark spots (cause of a structural change of an organic film).
[0128] And as the third advantage of the present invention, in case
of forming an organic EL element utilizing TFT or the like on the
same substrate, it is possible to reduce abnormal operations of TFT
due to diffusion of alkali metal being a material for the organic
EL element. Alkali metal or the like has a great thermal
diffusivity, and particularly is knocked on by heat during
formation of a film. The present invention can remove these
disadvantages by suppressing such diffusion.
* * * * *